instance method Array#indexOf

offset(Number)
– The number of initial items to skip before beginning the search.

Returns the index of the first occurrence of item within the array,
or -1 if item doesn't exist in the array. Array#indexOf compares
items using strict equality (===).

Array#indexOf acts as an ECMAScript 5 polyfill.
It is only defined if not already present in the user's browser, and it
is meant to behave like the native version as much as possible. Consult
the ES5 specification for more
information.